Many types of semiconductor fuses and antifuses are known in the art. Both the fuses and antifuses utilize the change in resistance of the individual structure under certain bias conditions. Once programmed, the programmed state of the fuses or antifuses does not revert to the original state on its own; that is, the programmed state of the fuse or the antifuse is not reversible. Thus, fuses and antifuses are conducive to the manufacture of a programmable read only memory (PROM). Programming or lack of programming constitutes one bit of stored information in fuses or antifuses. The difference between fuses and antifuses is the way the resistance of the memory element is changed during the programming process. Semiconductor fuses have a low initial resistance state that may be changed to a higher resistance state through programming, i.e., through electrical bias conditions applied to the fuse. In contrast, semiconductor antifuses have a high initial resistance state that may be changed to a low resistance state through programming.
Various methods of implementing antifuses in semiconductor structures have been disclosed in the prior art. In general, antifuses include one insulating layer sandwiched between two electrically conducting structures. In some cases, the insulating layer is a dielectric layer such as silicon dioxide, silicon nitride, or a stack comprising silicon nitride layers and silicon dioxide layers such as an oxide/nitride/oxide (ONO) stack. Most of the antifuses are built sequentially such that each of the three components of the antifuse, i.e., the first electrically conducting structure, the insulating layer, and the second conducting structure, is built one on top of another. In this case, the sequential building of the antifuse components results in a vertical structure for the physical implementation of an antifuse. By supplying a large voltage difference across the two electrically conducting structures, a dielectric breakdown is induced and a current path between the two electrically conducting structures is formed, whereby the high resistance state of the antifuse changes to a low resistance state. Various materials may be used for each of the two electrically conducting structures. Improvements upon the basic structure are known in the prior art. As one example, U.S. Pat. No. 6,853,049 utilizes a silicide for one electrically conducting structure and polysilicon for the other electrically conducting structure. As another example, U.S. Pat. No. 6,750,530 provides a mechanism for lowering the antifuse programming voltage by providing a resistive heating element adjacent to, but not in contact with the antifuse.
Other antifuses utilize a layer other than a dielectric layer for the insulating layer. U.S. Pat. No. 5,272,666 provides one example of such a prior art where polysilicon is utilized as the insulating layer. Also, U.S. Pat. No. 4,914,055 discloses another prior art where amorphous silicon is utilized as the insulating layer. U.S. Pat. No. 6,512,284 discloses a structure where one of the two electrically conducting structures comprises a heater and two terminals for providing current through the heater.
Antifuses in the prior art typically require breakdown of dielectrics by electrical bias and/or heat. While these mechanisms offer reasonable reliability of operation, further improvement in the reliability of antifuse operation is desired. An antifuse element that provides improved reliability in programming is therefore desired. Furthermore, arrays of electrical antifuses and fuses require a sensing circuitry to sense the state of an individual antifuse element or fuse element. In the prior art, the sensing circuitry is typically built externally around an array of antifuses or fuses. This type of external sensing circuitry typically requires transistors of substantial size to handle large current and voltages through the wire connections of the array and to insure sufficient signal development during sensing operations. Incorporating a sensing mechanism into the antifuse structure would provide a more compact memory element with a less stringent requirement for current and voltage during sensing. Therefore, an antifuse structure with a built-in sensing mechanism is also desired.